This application claims priority from Japanese Patent Application No. 2001-212373 filed Jul. 12, 2001 and Japanese Patent Application No. 2001-266636 filed Sep. 4, 2001, the disclosures of which are incorporated by reference herein in their entireties.
1. Field of the Invention
This invention relates to an inclined magnetic field generation coil and a magnetic field generator which are used in a magnetic resonance imager (MRI).
2. Description of the Related Art
MRI is an imaging apparatus using a magnetic resonance phenomenon and is being extensively used for purposes of medical diagnosis and the like. Conventionally, normal conducting electromagnets, superconducting electromagnets and the like have been used for the generation of a magnetic field for MRI. However, as a result of the recent development of high-performance rare-earth permanent magnets, it has become a major trend to use rare-earth permanent magnets (hereinafter referred to briefly as xe2x80x9cpermanent magnetsxe2x80x9d) for the generation of a magnetic field for MRI, for example, in MRIs having a low magnetic field of 0.5 T or less.
A conventional magnetic field generator for MRI and the pole pieces and other components used therein are described below with reference to FIG. 10. FIG. 10 is a cross-sectional view of a magnetic field generator for MRI as viewed from the side. In FIG. 10, plate-like yokes 101 and 102 are supported by a columnar yoke 103. On this pair of plate-like yokes 101 and 102, generally discoidal permanent magnets 104 and 105 selected from the group consisting of Ndxe2x80x94Fexe2x80x94B, Smxe2x80x94Co and Smxe2x80x94Nxe2x80x94Fe magnets are disposed so as to face each other. Moreover, pole pieces 106 and 107 having a circular base are attached to the opposed faces of permanent magnets 104 and 105, respectively.
Permanent magnets 104 and 105 are each magnetized along the thickness, keeping the direction of magnetization from being antiparallel.
On the other hand, peripheral projections 106b and 107b are provided along the periphery of pole pieces 106 and 107 (i.e., the periphery of their bases), respectively. These peripheral projections 106b and 107b serve to produce a magnetic field having a uniform strength substantially in the center of the space between pole pieces 106 and 107, and these projections have an approximately constant height. Bases 106a, 107a and peripheral projections 106b, 107b are formed, for example, of a soft magnetic material laminated onto a soft iron material such as low-carbon steel or pure iron.
In the recesses on the gap side of pole pieces 106 and 107, a pair of inclined magnetic field generators 108 and 109 are disposed in order to produce an inclined magnetic field on the gap side of the opposed pole pieces. The main purpose of this inclined magnetic field generator is to act on the uniform magnetic field space on the gap side of the pole pieces and thereby disturb the uniformity of the magnetic field linearly by intention. Then, if NMR signals including the nonuniform magnetic field are received, spatial information can be added during image formation from the signals. In an ordinary magnetic field generator for MRI, three inclined magnetic field coils arranged orthogonally so as to coincide with the X-axis, Y-axis and Z-axis of a three-dimensional coordinate system. The technology of inclined magnetic field coils is described, for example, in Japanese Patent Provisional Publication No. 63-65848/""88.
Now, referring to FIG. 11 which is an enlarged cross-sectional view of the pole piece regions shown in FIG. 10, the construction of the pole piece regions on the gap side of the inclined magnetic field generator is described in greater detail. For example, on the gap side of inclined magnetic field generators 108 and 109, RF wave transmitters (also called transmission coils) 110 and 111, magnetic field regulation mechanisms 112 and 113, a subject carrying table 114, and the like are usually mounted. Thus, the effective gap length of an actual magnetic field generator for MRI is expressed by the following equation (1)
Lg=L0xe2x88x92(2xc3x97Ts+Tt)xe2x80x83xe2x80x83(1)
or by the following equation (2)
Lg=L1xe2x88x92{2xc3x97(Ts+Tb)+Tt}xe2x80x83xe2x80x83(2)
The symbols in these equations are defined as follows.
Lg: Effective gap length.
L0: Gap-side distance between peripheral projections 106b and 107b. 
L1: Gap-side distance between inclined magnetic field coils 108 and 109.
Ts: Thickness of transmission coil
Tt: Thickness of subject carrying table
Tb: Thickness of magnetic field regulation mechanism.
These equations (1) and (2) indicate that the space available for the imaging of a subject is gradually narrowed by the mounting of various component parts.
The aforesaid magnetic field regulation mechanisms 112 and 113 disposed on the gap side of the inclined magnetic field coils serves as a tool for making a final magnetic field adjustment during installation in a hospital or the like. For example, each of them comprises a resin plate having about 10 to 200 magnetic field regulating holes 117 formed therein, as illustrated in FIG. 12. By inserting any desired magnet pieces or magnetic material pieces (e.g., iron pieces) into the regulating holes, the magnetic field uniformity on the gap side of the pole pieces can be enhanced. However, since the number of holes is large and a high machining accuracy is required, the magnetic field regulation mechanisms are regarded as one of the cost-raising factors among various component parts of the magnetic field generator for MRI.
It is very important that magnetic field generators for MRI, including permanent magnet type ones, facilitate the imaging and diagnosis of subjects, though it naturally depends on the intended purpose. To this end, it is first required to widen the effective gap between the opposed pole pieces as much as possible. For example, in the case of the magnetic field generator illustrated in FIG. 10, it would be highly desirable to reduce the thicknesses of the component parts assembled into the pole pieces, because this enables efficient utilization of the effective gap. In practice, however, the permanent magnets, the pole pieces, the inclined magnetic field coils and the magnetic field regulation mechanisms are stacked as illustrated in FIG. 11, so that the gap is narrowed to hinder the production of a uniform magnetic field and, moreover, the space available for imaging purposes is limited.
Accordingly, an object of the present invention is to provide a magnetic field generator for MRI in which, by simplifying the component parts of the pole pieces incorporated therein, the magnetic field generator for MRI can be manufactured at a lower cost without detracting from its magnetic characteristics.
That is, the present invention provides an inclined magnetic field generation coil for use in a magnetic field generator for MRI, the coil comprising a plurality of magnetic field regulating holes, an electric conductor in coiled form, and a resin base. Moreover, it also provides a magnetic field generator for MRI comprising a pair of permanent magnets magnetized in the direction of the thickness and opposed to each other with a gap left therebetween, and a pair of pole pieces having a peripheral projection and disposed on the opposed faces of the respective permanent magnets, in order to create a uniform magnet field space between the pole pieces, the magnetic field generator further including an inclined magnetic field generator disposed within each of the pole pieces and having a mechanism for regulating the magnetic field nonuniformity of the uniform magnet field space.
As described above, the present invention permits omission of the conventional magnetic field regulation mechanisms because a magnetic field regulation mechanism is provided in the surface, or in the interior, of the inclined magnetic field generator (i.e., the inclined magnetic field generation coil). This makes it possible to reduce the cost of the magnetic field generator for MRI and also to increase the effective gap length and thereby produce a uniform magnetic field.